The present invention relates to a method and an apparatus for surface sterilising items and a plant suitable for sterilising bottles.
From DE patent publication No. 40 18 142 a method is known for filling containers such as bottles, where microwaves are applies to the containers before filling for sterilising the containers. Humidity present on the surfaces of the containers evaporates, and the heat developed by the vapour kills undesired microorganisms on the surfaces. Before or during the treatment with microwaves, water may additionally be applied to or injected into the containers; thereby vapour is created due to the treatment with microwaves, said vapour killing the microorganisms. The development of steam is stated to take place in the course of a few seconds.
From the correspondence relating to the prosecution of Danish Patent Application No. 2354/89 a microwave packaging sterilising system for washing and sterilising plastic packagings is known, in particular for returnable bottles of polymeric plastic. In this system a washing of the packaging takes place at a temperature not exceeding 35-40xc2x0 C. to prevent shrinking of the packaging. To obtain, with certainty, a visual cleanness of the packagings at this low temperature, ultrasound is used, making it possible to avoid addition of lye to the washing water.
The washed packaging is not sterile after washing at such a low temperature, and therefore a sterilising using microwaves is carried out after the washing.
It is stated in the correspondence as a fact that the packaging would be heated to a temperature above deformation temperature if it was subjected to microwaves in a wet condition. This is explained by the fact that high amounts of energy would be transferred to the remaining moisture in and on the packaging. To prevent the application of microwave from heating the plastic material above the deformation temperature, it is therefore stated in the correspondence that during the sterilising treatment with microwaves an exact control of the microwave energy should be exerted as well as a removal of moisture from the packaging by means of a drying and cooling flow of sterile air.
It is, however, not described in the correspondence how this control of the microwave energy is to be carried out. It is only stated that the packaging can endure big amounts of microwave energy when it is dried, as it has a crystalline crystal lattice structure and consequently is not influenced by the microwave energy which exclusively kills bacteria and spores because they are xe2x80x9corganicxe2x80x9d.
There is no detailed description in the correspondence of how the system is to be operated or worked in order not to heat the packaging in an inappropriate way and to in fact kill the microorganisms effectively, and it seems doubtful if a full sterilisation would be obtainable at all by following the instructions given in the correspondence.
Further, Swedish patent publication No. 462.281 discloses a method and an apparatus for sterilising empty packaging containers by application of steam under pressure, preferably at a temperature of 120-140xc2x0 C., and preferably by additional use of hydrogen peroxide. Then microwaves are applied to the containers, which makes the temperature rise to around 145xc2x0 C.
From Swedish patent publication No. 465.512 a method is known for sterilising a width of packaging material by spraying on a sterilising liquid such as hydrogen peroxide solution, which is then removed from the width of material by applying infrared light, microwaves, high-frequency energy or hot air.
Finally, German patent publication No. 34 14 268 discloses a method and an apparatus for killing germs in food containers, like for instance cups for dairy products, by applying hydrogen peroxide to the containers, the peroxide being atomized by means of ultrasound at a frequency above 1 MHz. At the end of the process the containers are subjected to a thermal treatment by means of hot air or microwave.
None of the methods described in these three publications may be used for sterilising heat sensitive items.
The object of the invention is to provide an effective surface sterilising of items which are penetrable by high-frequency electric energy, in particular items which do not withstand heat exceeding a certain limit. It is part of the object that a definite part of the surface of the item is to be sterilised. This part will often be the whole surface of the item, but if the item for instance is a container, the surface in question may be the whole interior surface of the item.
Important examples of said items are plastic containers which have been shaped by blow moulding or vacuum moulding in hot condition, for instance the commonly used returnable bottles for soft drinks made from PET (polyethylene terephthalate) or the likewise commonly used flat trays of vacuum-shaped PE (polyethylene) film, PS (polystyrene) film or the like for packing for instance meat products in self-service stores. Such containers often possess a so-called elastic memory or xe2x80x9cresidual activityxe2x80x9d, by which is to be understood the phenomenon that the item may xe2x80x9crememberxe2x80x9d a shape and a size, which it had at an earlier stage of manufacture, and that it often is liable to return to this shape when heated.
Moreover, the items may for instance comprise containers for packing preserves or semipreserves (for instance translucent containers with screw lids for marmalade, herrings, etc.) or for packing non-preserved technical products like unpreserved plastic paint.
Said soft drink bottles are case as a so-called preform, i.e. a bottle in which the mouth area with thread etc. has its final size and shape, but in which the container part is quite small and thick-walled. These preforms take up very little room and may therefore at a low cost be transported from a central place of manufacture to various finishing places, often in other countries, where they are blown up in a hot condition for the container part to take up the desired shape and size. The container part may then be given a desired, individual shape according to the wish of the soft drink producer without the need for using different preforms.
It has turned out that these soft drink bottles tend to shrink on account of said elastic memory, already at temperatures slightly above 40xc2x0 C. Therefore, known washing and sterilising processes are not suitable for cleaning such bottles, as the bottles are liable to shrink at the temperatures used in these known processes, typically around 60xc2x0 C. or above. Such shrinking is obviously undesirable, and a major part of the bottles have to be discarded after having been cleaned a few times using the known processes.
At the same time there is a considerable need for an effective cleaning and sterilising of such bottles, as partly the owners (the soft drink producing factories) want them to be refilled with differing products without any preceding sorting, which, as evidenced by experience, may give a distaste from the previously contained product, and partly the users after emptying the bottles notoriously use them for the most differing purposes, like for instance storing of petrol or diesel oil, or refilling with different strongly tasting beverages, like a Danish fashion drink xe2x80x9csm{dot over (a)} gr{dot over (a)}xe2x80x9d (xe2x80x9cSmall Gray Onesxe2x80x9d; liquorice-and-ammonium-chloride-drops dissolved in vodka).
To meet the above object the present invention discloses a method, an apparatus and a plant.
The desired sterilising is obtained by heating a liquid film, and the heating is carried out by subjecting the item to high-frequency electric energy.
Experiments have shown that it is extremely important that the liquid to be heated by the high-frequency electric energy is actually present on the item in the form of a liquid film. Only in this way is the generation of a sterilising amount of heat ensured over the entire desired surface, without the heat development becoming so intense that an undesirable heating of the item itself takes place.
By providing the liquid film it is thus ensured that a substantial heating of the micro-climate on the surface of the item actually takes place. Besides, a total drying of microorganisms and germs on the item is avoided, whereby is ensured that they are optimally sensitive to high-frequency electric energy. Finally, the risk of sporogenous microorganisms creating spores is reduced, which will normally happen if these organisms are dried out.
Due to the fact that the liquid is present on the item just in the shape of a film, it is, as mentioned above, ensured that the amount of liquid is so small that during heating it cannot transfer sufficient heat to the item to heat the latter significantly.
By the liquid film established on the item being continuous, a sterilising is ensured of the surface of the entire item, or of a corresponding, definite part of the surface in its entirety, respectively.
According to the invention, the formation of the substantially continuous liquid film is ensured by using a liquid which is not repelled by the material of the item. Particularly for uses relating to food, water is preferred, which is, however, repelled by far by most polymeric materials. According to the invention, in this case a de-tensioning agent may be added. This also gives the additional advantage that the thickness of the liquid film to a certain extent may be controlled by controlling the type and the concentration of the de-tensioning agent. As de-tensioning agent may be used salts like for instance soda, organic solvents like for instance ethanol or surface-active agents. In particular surface-active agents are preferred, particularly the non-ionic type, as these may be used in lower concentrations and do not occasion substantial formation of foam. By the measures mentioned it is obtained that the liquid film is established in a particularly simple way.
The method according to the invention may advantageously be used for packagings, including returnable bottles and packaging trays for food. This is due to the fact being that the method according to the invention makes it possible to avoid using washing and sterilising chemicals, which is firstly undesirable in connection with packagings for foods and secondly considerably less detrimental to the environment.
By heating to the boiling point a particularly effective and quick sterilising is obtained.
By high frequency energy after boiling is obtained the effect that any water-containing microorganisms which may be left after the removal of the liquid film, are killed.
By removing excess energy is obtained the effect that an appropriate loading of the microwave source, normally a magnetron, is taking place, even though the items with the liquid film present do not constitute any noticeable load.
By using certain frequencies mentioned a comparatively high electric power may be used, as in most countries particularly easy rules apply for the use of these frequencies. A particularly simple tool is obtained for transferring high-frequency energy to the items.
The same liquid which has been used for washing or rinsing the item may be used for the formation of the liquid film.
By using ultrasound a particularly effective washing is obtained, which is a prerequisite for the adherence of the liquid film and for impurities not being present on the item, which impurities would be able to break the liquid film.
By designing the apparatus with dripping off means in the form of transport means, required transport means are utilised also as dripping off means. It is furthermore obtained that in a simple and safe way a substantially continuous liquid film is left on the items without any risk of the film drying out.
By blowing across the surface blowing off of liquid is ensured which may have accumulated in cavities on the item and/or a desired smoothing of the liquid film and/or a desired cooling of the item prior to the treatment with high-frequency energy.
By constructing a plant as described, the rinsing liquid used in the rinsing section may be used straight away for the formation of the liquid film for use in sterilising, whereby this liquid serves a double purpose. Thereby, a most compact system is obtained together with a washing and sterilising process, which is optimally gentle towards the bottles in respect of temperature stress on the material of the bottles. This system is suitable for sterilising both returnable bottles and throwaway bottles.
By providing a washing section the sterilising may be carried out immediately following washing of the bottles. When ultrasound is employed, the washing may be carried out with a minimum use of lye for quite without lye and thus as little detrimental to the environment as possible. Such a washing will first of all be necessary when the system is being used for returnable bottles.